Molded table and its method of manufacture

ABSTRACT

A plastic table structure includes a tabletop comprising a thin polymer shell layer surrounding an expanded foam core. The multiple layers of the finished tabletop are molded concurrently in a one pass biaxial or centrifugal rotational molding process, and cured in an oven. The rotational molding process advantageously allows the shell and foam core polymer materials to be distributed into all portions of the mold. In one embodiment, a structural frame is disposed in the mold, and the foam core encases and surrounds the frame. The structural frame may be made of steel, wood, composite materials, or other materials.

[0001] The present application claims priority from U.S. provisional patent application Ser. No. 60/389,855, filed on Jun. 18, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to molded tables. More particularly, the present invention relates to a molded plastic table structure having multiple concurrently molded layers.

[0004] 2. Related Art

[0005] There are a variety of folding tables, including folding tables with plastic tabletops, such as are used in hotels, churches, meetings, parties, temporary household use, and other uses. Typical folding tables have folding legs that fold against the underside of the tabletop, to enable easy moving and storage, while minimizing the storage space required.

[0006] The tops of prior art folding tables are usually fabricated of particleboard or laminated plywood, and may include a hard surface facing on the top. These tables typically incorporate a steel frame and folding steel legs. Unfortunately, these tables present a number of common drawbacks. The edges of metal frame members can be sharp and injure the fingers of people handling the tables. Particleboard and plywood tops tend to break easily if dropped, and are susceptible to moisture, which, if absorbed, may damage the tabletop, or discolor the surface. These tops also tend to bend and flex excessively when too much weight is placed on the table, and attempts to strengthen them tend to merely add to bulkiness and weight.

[0007] To overcome some of these problems, folding tables with an aluminum tabletop have also been fabricated. Unfortunately, these tables are relatively expensive, and are also susceptible to undesirable wear and tear, producing dents and sharp edges.

[0008] Plastic materials have recently come into use for the fabrication of lightweight folding tables, including plastic tables with lightweight cores, having plastic layers or grid frameworks as reinforcing members with plastic layers in various forms. Prior plastic tables are typically fabricated by forming a skin, such as by blow molding, rotational molding, or vacuum forming. This tends to create a weak shell. A frame may be disposed within the shell or connected to the exterior of the shell to add structural rigidity, and a plastic foam material, such as polyurethane foam, may also be injected into the shell to increase the stiffness of the tabletop. This method produces a relatively good finished plastic table structure. However the number of steps and secondary processes required to form such a table are costly and time-consuming for the producer. For example, injected foams produce relatively high fluid pressures (e.g. 40-50 psi), thus requiring a very strong support structure or mold to contain the hollow skin when the foam core is injected and expands. Consequently, injected foams must be applied in a step separate from the table molding step because thick heavy molds capable of withstanding the post-foaming pressures are too heavy and bulky for rotational molding processes. Such molds would be thermally inefficient, and would impose impractically large loads on rotational molding equipment. Additionally, because the foam core is formed through injection, there is a substantial likelihood of delamination of the skin from the foam core. Finally, injected polyurethane foams are relatively expensive.

SUMMARY OF THE INVENTION

[0009] It would therefore be advantageous to have a method for producing a lightweight reinforced plastic table structure which includes fewer steps and fewer secondary processes.

[0010] It would also be advantageous to have a lightweight reinforced plastic tabletop which can be fabricated in one pass through a molding process It would also be advantageous to have a method for producing a plastic table structure which does not involve high pressures associated with injected foam materials.

[0011] It would also be desirable to have a method for producing a lightweight plastic table structure which produces a tabletop having an integrated structural frame.

[0012] It would also be advantageous to have a plastic table with a foam core which is less susceptible to delamination of the plastic skin from the foam core.

[0013] The invention advantageously provides a tabletop, comprising a rotationally-molded polymer shell, and an expanded foam core, disposed within and substantially filling the shell, and which is expanded within the polymer shell during the rotational molding of the shell. In one embodiment, a structural frame is disposed within the shell, and the foam core substantially surrounds the structural frame.

[0014] In accordance with another aspect of the present invention, the invention provides a method of rotationally molding a tabletop. The method includes the steps of placing polymer material within a tabletop mold that is part of a rotational molding system, and heating the mold while rotating it, such that the polymer material forms a shell on the inside of the mold. An expansive polymer material is introduced into the shell, so as to form an expanded foam core inside the shell and extending into substantially all interior regions of the shell. The mold is then cooled (usually while continuing to rotate it) to allow the tabletop to be removed therefrom.

[0015] In another more detailed embodiment, the method includes the step of placing a structural frame within the mold, so that the foam core surrounds the structural frame within the shell.

[0016] Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a cross-sectional view of a lightweight tabletop fabricated according to the present invention.

[0018]FIG. 2 is a cross-sectional view of a mold having a frame disposed therein prior to formation of the tabletop therearound.

[0019]FIG. 3 is an elevation view of a rotational molding system configured for forming the table of the present invention.

[0020]FIG. 4 is a pictorial view of a table made in accordance with the present invention.

[0021] FIGS. 5-7 are partial cross-sectional views of alternative tabletop structures in accordance with the present invention.

[0022]FIG. 8 is an edge view of a tabletop according to the present invention, showing the shrink-neutral axis and possible shrinkage-related deformation of the table.

[0023]FIG. 9 is a partial cross-sectional view of a tabletop in accordance with the present invention, incorporating an embedded frame member having its neutral axis coincident with the shrink-neutral axis of the tabletop.

DETAILED DESCRIPTION

[0024] Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the invention as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.

[0025] The invention advantageously provides a plastic tabletop structure that is fabricated in a mold in one pass. The method produces a multiple layer laminate which provides greatest material placement into the weakest areas. It also allows production of a layered laminated foamed core with multiple densities, all produced in a one step molding process. Advantageously, the plastic tabletop keeps its shape because it remains in an encapsulated mold while cooling. Additionally, the table skin resists delamination because the skin and foam are of the same polymer family and integrally bond while at elevated temperatures.

[0026] Viewing FIG. 1, the table 10 of the present invention comprises a rigid frame 12, such as a tubular steel frame, encased within a polymer foam material 14, such as expanded polyethylene foam, which is in turn encapsulated in a plastic skin 16, such as a more dense polyethylene material. As shown in FIG. 1, the frame may be incorporated into a skirt 18 which extends downwardly from the tabletop. Advantageously, the tabletop with all of these elements is completely formed in a mold in a single step, thereby eliminating the secondary fabrication steps normally employed in making such tables. It also produces a very strong table which is durable and resists delamination of the skin from the foam core and the frame.

[0027]FIG. 2 shows a cross-sectional view of a mold assembly 20 for producing the table of the present invention. The mold may be manufactured from a metal or other material, such as cast aluminum, or fabricated sheet aluminum, or other suitable cast or composite materials, such as steel, iron, etc. Cast aluminum is presently preferred because of its good balance between cost and heat transfer characteristics.

[0028] Within an inner cavity 22 of the mold 20, and attached to the inside of the walls thereof, are a series of pins 24 for supporting or suspending the structural frame 12 within the mold prior to and during the molding process. The ends of the pins protrude into the mold, and during the molding process, hold the structural frame in place as it becomes encapsulated by the skin and expanded foam material, as will be explained in more detail below. The pins may be of metal, and may be adjustable or removable from outside the mold. Alternatively, the pins may be of a polymer material which becomes part of the tabletop during the heating and molding process.

[0029] It will be apparent that the frame 12 may be supported within the mold in other ways, the results of which are shown in two embodiments in FIGS. 6 and 7. The frame may be supported within the mold cavity 22 by a leg frame attachment plate 26 (FIG. 6), or by bolt sockets 28 (FIG. 7) or other mechanical fastener-related structures which extend to or through the mold walls and serve the same function as the pins. This method allows for insert-molding of fastener systems, whether attached to the internal frame, or encapsulated within the shell 16, or within the integrally molded polymer core 14. The fasteners or other inserts allow for the attachment of leg systems 30, and other accessories to the plastic table structure. Other mechanical supports to suspend the frame in the mold may also be used.

[0030] Returning to FIG. 2, incorporated into the walls of the mold 20 are breather tubes 32, which allow gasses—from reactions of the polymer resins in the inner cavity of the mold during the molding process—to escape to the atmosphere. The breather tubes also serve to equalize pressures that could build up in the inner cavity of the mold during heating and cooling. While breather tubes are shown on both the top and bottom of the mold in FIG. 2, it will be apparent that breather tubes may be placed only on the top or only on the bottom.

[0031] Disposed on the outer periphery of the mold 20 is a canister or drop box 34 that can be pneumatically, electrically, or hydraulically opened. While the drop box is shown attached to the portion of the mold corresponding to the table top, it will be apparent that it could also be attached to the bottom of the mold, and the inventors have practiced the invention in that configuration. Drop boxes are well known in the art of rotational molding. The drop box is configured to hold a supply of one or more raw polymer materials 36, which are allowed to “drop” or flow into the mold at a set time (or temperature) during the heating and/or cooling process. The drop box is mounted on the outer periphery of the exterior mold surface, with an access hole 38 provided from its interior chamber to the inner cavity 22 of the mold. The drop box includes a plunger 40, which normally blocks the access hole, but when actuated, draws away from the access hole to allow the polymers stored inside the canister to flow into the inner cavity of the mold. As noted above, the plunger may be pneumatically, electrically, or hydraulically actuated. Furthermore, its actuation may be triggered electrically, through either a hard-wired connection, or a wireless radio frequency control system.

[0032] If desired, more than one drop box 34 may be attached to the mold 20 to allow more than one “drop” or discharge of material into the mold during the molding process. Likewise, a drop box with more than one chamber may be used for the same purpose, as depicted in FIG. 2. This drop box contains a first polymer 36 a, which may be, for example, polymer pellets of relatively small size, and a second polymer 36 b, which may be a polymer having larger sized particles. The walls 42 of the drop box are heavily insulated, and the materials surrounding the aperture 38 are selected to prevent adhesion of the contained polymer material thereto. The insulation allows the material contained in the drop box to remain at a lower temperature than the mold itself, for reasons which will become more apparent hereafter.

[0033] When fully prepared, the mold 20 is ready to be attached to the rotational molding machine 50 and placed within an oven 52, as shown in FIG. 3. The mold assembly is mounted on a frame 54, which is fixedly attached to the end of a rotatable shaft 56. The shaft is part of the rotational molding machine, and is driven to rotate about its longitudinal axis, in the direction shown by arrows 60, by a first mechanical power source 62, such as an electric motor. The first mechanical power source for the shaft in turn is mounted on a rotatable spindle 64, which has a longitudinal axis that is substantially perpendicular to that of the shaft. The spindle is rotatably mounted on a frame 66, and is rotationally driven by a second mechanical power source (not shown), such as an electric motor. The first and second mechanical power sources for the rotatable shaft and spindle, respectively, are configured to rotate their respective elements at speeds of anywhere from about 1 rpm to about 16 rpm, though other speeds may be used. In one embodiment, the inventors prefer a speed in the range of about 6 rpm to about 8 rpm. These components thus have the capacity to simultaneously rotate one or more molds about two orthogonal axes. This is typical of common rotational molding.

[0034] To produce the table disclosed herein, the mold assembly 20 is first mounted onto the frame 54 at the end of the rotatable shaft 56. The mold is then opened, and, depending on the desired combination of structural, physical and aesthetic properties desired, one or more of several procedures may be followed. In one embodiment, the inside surfaces of the open mold are first treated with a release agent, which allows the finished product to be easily removed from the mold. Suitable release agents include water based or solvent based silicones or Teflon. These and other suitable release agents are well known in the art, and are readily commercially available.

[0035] The structural load-bearing frame 12 is then inserted into the inner mold cavity 22. FIG. 4 shows a pictorial view of a completed table 70, with the internal frame shown in hidden lines. The structural frame may be fabricated from a variety of materials, including wood, metal, polymers, or composites. Polymers may be used for the frame so long as they are stable at and are not damaged by temperatures that will be reached during the rotational molding process. The frame is supported or suspended within the mold cavity on the pins 24 or mechanical fastener-related structures described above, which protrude into the mold. It will be apparent that a variety of frame configurations may be used. The frame typically will include a perimeter frame member 72, which is usually enclosed or encased within the edge skirt 18 of the tabletop, as shown in FIG. 1. However, a table in accordance with the present invention could be configured without an edge skirt, as indicated by the dashed line 75 in FIG. 5.

[0036] The frame 12 may also comprise one or more transverse frame members 74, as shown in FIGS. 2, 4, 6, and 7. These would presumably be connected to the perimeter frame 72, where there is one. Additionally, the table may include longitudinal frame members 76 as shown in FIGS. 4, 5, and 6. As with the transverse frame members, the longitudinal frame members would presumably be attached to the perimeter frame and transverse frame members, where these are present. It will also be apparent that the table may be configured without any internal frame members at all, or may have only longitudinal frame members, such as only in the edge skirt 18 on the long sides of a table. Alternatively, the table may have a frame that extends only around its perimeter. Many framed and unframed configurations are possible.

[0037] Referring back to FIG. 2, after insertion of the frame 12, raw polymer material, usually in the form of powder or pellets, is placed in the mold 20 for forming the thin polymer shell or skin 16 of the table. The exterior shell polymer may be of thermoset plastic or thermoplastic compounds, and may contain ultraviolet light inhibitors, anti-oxidants, reagents, or color additives as desired. This polymer material may be, for example, polyethylene, polypropylene, polyvinyl chloride, or composite polyester. Other materials may also be used. While the shell polymer material is usually in the form of powder or pellets, liquids may also be used, and may be sprayed onto the interior mold surface. The thin polymer layer forming the shell is intended to provide various desired properties, including color, texture, abrasion resistance, opacity, translucence, multiple color surfaces, impact resistance, and structural strength.

[0038] With the frame 12 and the polymer for forming the shell 16 in place in the mold, the mold is then closed. At this point, the drop box 34 is attached to the mold 20, having its aperture 38 in line with a corresponding aperture in the mold, and one or more raw polymer materials 36 in the form of powder or pellets are placed into the drop box(es). An actuator 44 is attached to the drop box to control operation of the plunger 40, to allow the contents of the drop box to be introduced into the mold at the proper time.

[0039] After all of the raw materials have been placed in the mold 20 or in their respective drop box(es) 34, the mold assembly is mounted on the rotational molding machine 50 and placed in the oven 52. As the mold rotates, the polymer for forming the skin is caused to spread out within the mold. Simultaneously, the oven, having heating elements 66, heats the mold, which causes the polymer particles to begin to melt and adhere to the inner surface of the mold. The result of the heating and rotating is to form an exterior shell of the melted first polymer around the entire inner surface of the mold.

[0040] At a preset time or temperature, the drop box 34 opens, allowing the second polymer to flow into the mold. The second polymer preferably contains reagents that will cause it to “blow” or expand into the foam core 14 in a controlled manner at a predetermined temperature. This temperature may be approximately the same as the temperature at which the skin 16 forms. However, because the drop box is thermally insulated, the second polymer will not have reached that temperature by the time the first or shell polymer does. Consequently, the same material, e.g. polyethylene, may be used for both the shell and the foam core, the only difference being that the polymer of the core includes the blowing agent so as to expand into a foam, while the shell polymer does not. Because of the timing of their exposure to the reaction temperature, the desired reactions will occur at different times.

[0041] Many different kinds of foams may be used. For example, two kinds of olefinic foams have been used by the inventors. Azodicarbonamide foams produce nitrogen gas (N₂) and carbon dioxide (CO₂), as the blowing agents, but also produce ammonia (NH₄) and carbon monoxide (CO) as byproducts. Obviously, carbon monoxide is poisonous, and ammonia has an objectionable smell, and is also toxic in large quantities. Alternatively, sodium bicarbonate-based foams have also been used, these producing carbon dioxide (CO₂) as the blowing agent, with no objectionable byproducts. This latter method is preferred. Through this process, two similar (or perhaps even dissimilar) materials, the skin polymer and the foam polymer, form a laminate which becomes integrally connected into a strong mass. When viewed in cross-section and on a magnified scale, the unexpanded material of the shell 16 gradually transitions into the expanded foam material of the core 14, such that there is no distinguishable interface between the two materials. To the naked eye, the transition from the non-expanded shell to the expanded foam core material does not appear gradual. However, because the core material and shell material are placed and cured together and may be the very same type of material, the transition from one to the other primarily represents a change in density, rather than an interface between two materials. Consequently, there is no weakened interface between the shell and the core, thus greatly reducing the problem of delamination of the skin from the foam core, even when subjected to heat and other stress.

[0042] One advantage of this method is that olefinic foams are substantially less expensive than injected foams, such as polyurethane isofoam. Thus, the method of this invention allows less expensive foam materials to be used for lightweight table cores which could not be used before. Olefinic foams also produce far less fluid pressure (˜5 psi) than injected foams (which produce ˜40-50 psi), thus allowing their use in relatively lightweight and less expensive rotational molds. The “blowing” or foaming reaction of sodium bicarbonate-based foams is an endothermic reaction. The inventors have found that the use of an endothermic foaming agent can make temperature control easier during the rotomolding process. However, exothermic foaming agents can also be used in accordance with the method of this invention.

[0043] Many “drops” of polymer materials, colors, or reagents may be made into the mold cavity as desired, whether from a single drop box having more than one chamber (as in FIG. 2), or from multiple drop boxes (not shown). For example, referring to FIG. 7, after the first polymer material is allowed to form the shell 16, a second shell polymer material may be dropped into the mold, to form a second shell layer 80 inside the first. Thus one or more additional layers of polymer may be deposited inside the outer shell layer 16. The second and subsequent layers of polymers are preferably of such a characteristic that each layer will mold, in sequential order, after the primary shell has been formed.

[0044] During the heating cycle, the polymer pellets may be of various sizes, each size melting and reacting at different times. In general, the smaller the pellet, the faster the melt—similar to a time-release system. The heating cycle heats the mold and its contents from room temperature up to a certain maximum temperature, depending on the specific properties of the polymer materials that are being used. In one embodiment of the invention, using polyethelyne for the shell material, the temperature at which the shell begins to form is about 270° F., and the temperature at which the foam core forms is about 310° F. However, with other materials, the temperatures will differ. The melt temperature of nylon, for example, whether for the shell or the foam core, is between about 347° F. and 509° F.

[0045] The inventors have also found that a variety of different materials can be placed into the mold at the beginning of the process (without using a drop box) and still produce the different layers. Because the different materials have different properties, they can form successive layers of the table, including both shell materials and core materials even while intermixed. For example, each shell layer material may have a slightly different melt temperature, such that they will melt and adhere to the inside of the mold (or the preceding material) at different times during the molding process.

[0046] The maximum temperature may be maintained for some period of time to allow the desired reactions to go to completion, or upon reaching the desired temperature, the heating cycle may be immediately discontinued. In one embodiment of the invention, the heating cycle lasts approximately 25 minutes. When the heating cycle is completed, the mold assembly 20 is removed from the oven 52, and placed in a cooling area (not shown) for a given time period. In one embodiment of the invention, the cooling cycle lasts for about 25 minutes. While the mold is cooling, additional material drops may be made in the inner cavity of the mold. After cooling, the molded part is removed, and the process can be repeated.

[0047] The method as described produces a unique plastic table structure. The plastic table structure utilizes a combination of a foam core 14, encapsulated within a polymer shell 16 having one or more layers, to produce a plastic table that is very strong and has high impact resistance. Advantageously, the foam core and polymer skin may be of the same species of material, simply in different forms or densities (i.e. foam vs. higher density skin), thus providing an integral transition from the core to the skin, and thereby drastically reducing the possibility of delamination. The unique concurrently molded polymer core system produces a solid platform that resists crushing and also inhibits ultraviolet degradation.

[0048] The inventors have found that in mechanical stress tests, the shell and core advantageously appear to function as a single structural unit. For example, a tabletop according to the present invention has been tested for pull-out strength with respect to mechanical connectors embedded through a bottom layer of the shell and into the foam core, but not extending to the top layer of the shell. When a force great enough to pull the mechanical connectors from the table was applied, a section of the bottom shell layer and foam core in the region of the connectors broke away from the table. This was expected. Unexpectedly, however, a substantial portion of the top shell layer also broke away from the table, and remained attached to the broken-away piece of foam core. This test demonstrated that the bond between the foam core and the higher density shell material is mechanically stronger than the shell material itself.

[0049] The table structure can also be modified with a variety of cosmetic and functional features. For example, inserts of various kinds (not shown) can be placed in the mold 20 before molding, so as to be incorporated into the finished table. These may include laminate inserts for the tabletop, protective edge bands, facia pieces, and the like. For example, a layer of ultra-thin Corian® or other durable laminate material could be placed into the mold to provide a tabletop that has superior surface qualities in an inexpensive polymer shell. This process could be used to produce things such as laboratory benches, and highly impermeable surfaces for use where granite and other such materials are currently used. It will be apparent that laminates and other such additions could also be applied to the finished tabletop after the molding process is complete.

[0050] One challenge presented by rotationally-molded articles is shrinkage and deformation after molding. As a rotationally-molded article cools down after formation, its material thermally shrinks. Naturally, this shrinkage induces internal stress in the article, and, depending upon the geometry of the article, this stress can cause significant deformation. When an internal frame is incorporated into a rotationally-molded article, this tends to further complicate shrinkage-induced deformation. For example, an internal frame is likely to have a different coefficient of thermal expansion, which will change the nature and magnitude of shrinkage-induced mechanical stress inside the structure. Whether the frame bonds to the internal foam core material will also affect the nature and degree of internal stress. These problems can cause additional warpage, or make the warpage more severe or difficult to predict.

[0051] Referring to FIG. 8, there is shown an edge view of a tabletop 110 according to the present invention, showing possible shrinkage-related deformation of the table. The table is geometrically irregular, having a large, planar tabletop 112, and a skirt 114 that is perpendicular to the tabletop and extends around the table perimeter on the bottom side. Because of this geometry, as the table cools and shrinks, the shrinkage stress in the tabletop causes it to warp, as shown by the dashed lines in FIG. 8.

[0052] One common method for dealing with warping or other undesirable deformation of rotationally molded articles is to modify the shape of the mold to anticipate potential warping. For example, to eliminate undesirable warping of a rotationally-molded tabletop, the top surface of the mold can be slightly curved, so that as the item cools, the natural shrinkage-induced warping will bring the tabletop to the desired flat shape. As shown in FIG. 8, the tabletop 110 shrinks relative to a shrink-neutral axis 116. This axis represents a plane within which shrinkage does not produce any warping or change in relative shape. Above and below the shrink-neutral axis, the relative shape of the table changes due to shrinkage. The location of the shrink-neutral axis depends upon the geometry of the article. Because certain portions of the structure have greater stiffness in the direction of shrinkage, warping will vary accordingly.

[0053] The inventors have found that warping can be reduced through proper attention to the placement of an internal frame member with respect to the shrink-neutral axis. Referring to FIG. 9, an embedded frame member 120 is placed with its neutral axis 122 coincident with the shrink-neutral axis 116 of the tabletop 110. This configuration helps prevent differences in thermal expansion between the frame and tabletop from causing additional warping. The bending stiffness of the frame member also helps reduce the normal warping that would occur if no frame member were present.

[0054] In one embodiment, the inventors have also found it desirable to use a frame member 120 that does not bond to the material of the foam core 124. The polymer tabletop 110 is likely to have a higher coefficient of thermal expansion than the frame member. If so, the tabletop will shrink more than the frame member as it cools. If the core material were bonded to the frame, this could induce significant additional shrinkage stress, and thus produce more warping. However, if the beam does not bond to the expanded foam core material, the foam material can “slide” along the beam as it shrinks, and only a small, localized region of foam material—such as adjacent to an end of a beam—may be deformed due to shrinkage, without causing significant additional warping.

[0055] By way of example, the invention could be described as a tabletop, comprising a rotationally-molded polymer shell configured as a tabletop, a structural frame, disposed within the shell, and an expanded foam core, disposed within and substantially filling the shell. The foam core substantially surrounds the structural frame, which is expanded within the polymer shell to fully surround the structural frame during the rotational molding of the shell.

[0056] As another example, the invention can be described as a tabletop, comprising a rotationally-molded polymer shell with an interior of less than 3 inches maximum thickness and more than two feet in length. A structural frame is disposed within the interior of the shell, and a structural foam core encases the structural frame and substantially fills all portions of the interior. The structure is such that the frame, foam, and shell form a single composite body to support and distribute physical loads, applied to substantially any portion of the polymer shell, to the structural frame.

[0057] As yet another example, the invention can be described as a tabletop, comprising a rotationally-molded shell of non-expanded polymer material, a structural frame, disposed within the shell, and a foam core of expanded polymer material, disposed within the shell and surrounding the structural frame. The foam core is expanded within the shell during the rotational molding of the shell. The shell and the foam core are formed of similar polymer materials, producing a cross-section of the tabletop that exhibits a gradual transition from the non-expanded shell material to the expanded material of the core, without a distinguishable interface therebetween.

[0058] As still another example, the invention can be described as a method of rotationally molding a tabletop. The method includes the steps of placing polymer material within a tabletop mold that is part of a rotational molding system, placing a structural frame within the mold, and heating the mold such that the polymer material forms a shell on the inside of the mold. Then, an expansive polymer material is introduced into the shell, so as to form an expanded foam core surrounding the structural frame inside the shell and extending into substantially all interior regions of the shell. The mold is then cooled (usually while continuing to rotate it) to allow the tabletop to be removed therefrom.

[0059] It is to be understood that the above-described arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. Thus, while the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, variation in size, materials, shape, form, function and manner of operation, assembly and use may be made without departing from the principles and concepts set forth herein. 

What is claimed is:
 1. A tabletop, comprising: a rotationally-molded structure, configured as a tabletop having a flat top surface, including a first shell of a first density, and a first expanded foam core, within the first shell, of a second density lower than the first density, the shell and the core being integrally formed of a common polymer material in a single rotational molding process, having a region of transition between the shell and the foam core which is a transition substantially only of density within the common polymer material.
 2. A tabletop in accordance with claim 1, further comprising a second polymer shell substantially surrounding the first shell, and being integrally formed with the first shell and first foam core during the single rotational molding process.
 3. A tabletop in accordance with claim 1, further comprising a structural frame, disposed within the shell and encased within the foam core material.
 4. A tabletop in accordance with claim 3 wherein the structural frame is encased within the foam core material without adhesion between the foam and the frame.
 5. A tabletop in accordance with claim 3 wherein the structural frame comprises frame members selected from the group consisting of metal frame members, polymer frame members, and wood frame members.
 6. A tabletop in accordance with claim 3 wherein the structural frame includes a tubular beam member.
 7. A tabletop in accordance with claim 3 wherein the tabletop has an elongate dimension parallel with the top surface, and the structural frame includes at least one elongate beam oriented in the elongate dimension.
 8. A tabletop in accordance with claim 3 wherein the tabletop has an elongate dimension, and the structural frame includes at least one elongate beam oriented transverse to the elongate dimension.
 9. A tabletop in accordance with claim 3 wherein the structural frame has a neutral axis, and the structural frame is disposed in the tabletop with the neutral axis substantially coincident with a shrink-neutral axis of the molded table, so as to resist shrinkage-related deformation of the tabletop.
 10. A tabletop in accordance with claim 3 further comprising attachment points disposed on an underside of the tabletop, configured to connect to support structure for the tabletop, the attachment points being directly attached to the structural frame.
 11. A tabletop in accordance with claim 1, further comprising attachment points disposed on an underside of the tabletop, configured to connect to table legs for the tabletop.
 12. A tabletop in accordance with claim 1, further comprising a plurality of vertically oriented table legs, attached to the tabletop, configured to support the tabletop above a floor.
 13. A tabletop, comprising: a rotationally-molded polymer shell, configured as a tabletop having a flat top surface; a structural frame, disposed within the shell; and an expanded foam core, substantially filling the shell and surrounding the structural frame, the foam core being expanded within the polymer shell during rotational molding of the shell.
 14. A tabletop in accordance with claim 13, wherein the structural frame is disposed within the foam core material without adhesion between the foam and the frame.
 15. A tabletop in accordance with claim 13, wherein the structural frame comprises frame members selected from the group consisting of metal frame members, polymer frame members, tubular frame members, and wood frame members.
 16. A tabletop in accordance with claim 13, wherein the structural frame has a neutral axis, and the structural frame is disposed in the tabletop with the neutral axis substantially coincident with a shrink-neutral axis of the molded tabletop, so as to resist shrinkage-related deformation of the tabletop.
 17. A tabletop in accordance with claim 13, further comprising attachment points disposed on an underside of the tabletop, configured to connect to table legs for the tabletop.
 18. a tabletop in accordance with claim 17, wherein the attachment points are directly attached to the structural frame.
 19. A method of forming a rotationally-molded tabletop, including the steps of: introducing a first polymer material within a tabletop mold that is part of a rotational molding system, the mold having the form of a tabletop with a flat top surface; heating and rotating the mold in the rotational molding system, such that the first polymer material forms a shell on an inside of the mold; introducing an expansive polymer material into the mold; causing the expansive polymer material to expand, so as to form a foam core substantially filling the shell, to form a tabletop; and cooling the mold to allow the tabletop to be removed therefrom.
 20. A method in accordance with claim 19, wherein the first polymer material and the expansive polymer material are polymers of different types.
 21. A method in accordance with claim 19, wherein the first polymer material and the expansive polymer material are of a same type of polymer material, so as to produce a tabletop having a region of transition between the shell and the foam core which is a transition substantially only of density within the common polymer material.
 22. A method in accordance with claim 19, wherein the first polymer material comprises particles, and the expansive polymer material comprises particles of a larger size than the first polymer material, such that the first polymer material will melt at an earlier point during the step of heating and rotating the mold.
 23. A method in accordance with claim 19, wherein the steps of introducing the first polymer material and introducing the expansive polymer material are performed substantially concurrently, before heating and rotating the mold.
 24. A method in accordance with claim 19, wherein the expansive polymer material is introduced into the mold after the first polymer material forms the shell.
 25. A method in accordance with claim 24, wherein the step of introducing the first polymer material is performed before heating and rotating the mold, and the step of introducing the expansive polymer material is performed during the step of heating and rotating the mold, and substantially after the first polymer material has formed a shell on the inside of the mold.
 26. A method in accordance with claim 24, wherein the step of introducing the expansive polymer material comprises introducing the expansive polymer material into the mold from a drop box attached to the mold.
 27. A method in accordance with claim 24, further comprising the step of introducing a second polymer material within the mold, substantially after the first polymer material has formed a shell on the inside of the mold, and before the expansive polymer material is introduced into the mold, such that the second polymer material forms a second shell on an inside of the shell formed by the first polymer material.
 28. A method in accordance with claim 27, wherein the second polymer material is introduced into the mold from a drop box attached to the mold.
 29. A method in accordance with claim 19, further comprising the step of placing a structural frame within the mold before heating and rotating the mold, so that the structural frame becomes encased within the foam core.
 30. A method in accordance with claim 29, wherein the structural frame includes a neutral axis, and the step of placing the structural frame within the mold further comprises placing the structural frame such that the neutral axis substantially corresponds with a shrink-neutral axis of the tabletop, so as to inhibit shrinkage-related deformation of the tabletop.
 31. A method in accordance with claim 19, wherein the expansive polymer material includes a foaming agent configured to cause a foaming reaction in the expansive polymer material at a set temperature, so as to form the foam core within the shell.
 32. A method in accordance with claim 31, wherein the foaming agent is selected from the group consisting of azodicarbonamide foams and sodium bicarbonate-based foams.
 33. A method in accordance with claim 19, wherein the step of cooling the mold is performed while continuing to rotate the mold in the rotational molding system.
 34. A method in accordance with claim 19, wherein the shape of the tabletop mold varies from an intended finished shape of the tabletop, such that shrinkage related deformation of the tabletop after cooling will cause the shape of the tabletop to substantially conform to the intended finished shape. 